|A honey locust in Washington state shows its fall color.|
The honey locust, Gleditsia triacanthos, also known as the thorny locust, is a deciduous tree native to central North America where it is mostly found in the moist soil of river valleys ranging from southeastern South Dakota to New Orleans and central Texas, and as far east as eastern Massachusetts. The species has become a significant invasive weed in other regions of the world.
Honey locusts, Gleditsia triacanthos, can reach a height of 20–30 m (66&–100 ft), with fast growth, and are relatively short-lived; their life spans are typically about 120 years, though some live up to 150 years. They are prone to losing large branches in windstorms. The leaves are pinnately compound on older trees but bipinnately compound on vigorous young trees. The leaflets are 1.5–2.5 cm (smaller on bipinnate leaves) and bright green. They turn yellow in the fall (autumn). Leafs out relatively late in spring, but generally slightly earlier than the black locust (Robinia pseudoacacia). The strongly scented cream-colored flowers appear in late spring, in clusters emerging from the base of the leaf axils.
The fruit of the honey locust is a flat legume (pod) that matures in early autumn. The pods are generally between 15–20 cm. The pulp on the insides of the pods is edible, unlike the black locust, which is toxic. The seeds are dispersed by grazing herbivores such as cattle and horses, which eat the pod pulp and excrete the seeds in droppings; the animal's digestive system assists in breaking down the hard seed coat, making germination easier. In addition, the seeds are released in the host's manure, providing fertilizer for them. Honey locust seed pods ripen in late spring and germinate rapidly when temperatures are warm enough.
Honey locusts commonly have thorns 3–10 cm long growing out of the branches, some reaching lengths over 20 cm; these may be single, or branched into several points, and commonly form dense clusters. The thorns are fairly soft and green when young, harden and turn red as they age, then fade to ash grey and turn brittle when mature. These thorns are thought to have evolved to protect the trees from browsing Pleistocene megafauna which may also have been involved in seed dispersal, but the size and spacing of them is useless in defending against smaller extant herbivores such as deer. Thornless forms (Gleditsia triacanthos inermis) are occasionally found growing wild and are available as nursery plants. Hybridization of honey locust with water locust (G. aquatica) has been reported.
Its cultivars are popular ornamental plants, especially in the northern plains of North America where few other trees can survive and prosper. It tolerates urban conditions, compacted soil, road salt, alkaline soil, heat and drought. The popularity is in part due to the fact that it transplants so easily. The fast growth rate and tolerance of poor site conditions make it valued in areas where shade is wanted quickly, such as new parks or housing developments, and in disturbed and reclaimed environments, such as mine tailings. It is resistant to gypsy moths but is defoliated by another pest, the mimosa webworm. Spider mites, cankers, and galls are a problem with some trees. Many cultivated varieties do not have thorns.
Despite its name, the honey locust is not a significant honey plant. The name derives from the sweet taste of the legume pulp, which was used for food by Native American people, and can also be fermented to make beer. The long pods, which eventually dry and ripen to brown or maroon, are surrounded in a tough, leathery skin that adheres very strongly to the pulp within. The pulp—bright green in unripe pods—is strongly sweet, crisp and succulent in unripe pods. Dark brown tannin-rich beans are found in slots within the pulp.
Honey locusts produce a high quality, durable wood that polishes well, but the tree does not grow in sufficient numbers to support a bulk industry; however, a niche market exists for honey locust furniture. It is also used for posts and rails since it takes a long time to rot. In the past, the hard thorns of the younger trees were used as nails while the wood itself was used to fashion treenails for shipbuilding.
The species is a major invasive environmental and economic weed in agricultural regions of Australia. The plant forms thickets and destroys the pasture required for livestock to survive. The thickets choke waterways and prevent both domestic and native animals from drinking and also harbour vermin. The spines cause damage to both people and domestic and native wildlife and puncture vehicle tyres. In much of the Midwest of the United States the honey locust is also considered a weed tree and a pest that establishes itself in farm fields. In other regions of the world, ranchers and farmers who employ monocropping deem honey locust as a nuisance weed; its fast growth allows it to out-compete grasses and other crops.
The ability of Gleditsia to fix nitrogen is disputed. Many scientific sources clearly state that Gleditsia does not fix nitrogen. Some support this statement with the fact that Gleditsia does not form root nodules with symbiotic bacteria, the assumption being that without nodulation, no N-fixation can occur. In contrast, many popular sources, permaculture publications in particular, claim that Gleditsia does fix nitrogen but by some other mechanism.
There are anatomical, ecological and taxonomic indications to indicate nitrogen-fixation in non-nodulating legumes. Both nodulating and non-nodulating species have been observed to grow well in nitrogen-poor soil with non-nodulating legumes even dominating some sites. The litter and seeds of non-nodulating species contains higher nitrogen than non-legumes and sometimes even higher than nodulating legumes growing on the same site. How this happens is not yet well understood but there has been some observations of nitrogenase activity in non-nodulating leguminous plants including honey locust. Electron microscopy indicates the presence of clusters around the inner cortex of roots, just outside the xylem, that resemble colonies of rhizobial bacterioids. These may well constitute the evolutionary precursors in legumes for nitrogen fixation through nodulation. It is not known whether the non-nodulating nitrogen fixation, if it exists, does benefit neighboring plants as is said to be the case with nodulating legumes.
The tree has been used in traditional Native American medicine. Extracts of Gleditsia possess important pharmacological activities in treating rheumatoid arthritis, as anti-mutagenic, anticancer and have significant cytotoxic activity against different cell lines. Seeds of Gleditsia triacanthos contain a trypsin inhibitor.
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